Mitochondrial Thermostat

How well you cope with cold winter weather may depend on where your ancestors lived. Researchers now say that people from tropical, temperate, and arctic climes differ in how efficiently they produce energy and heat their bodies. The key to this efficiency lies in the genetic code found in mitochondria, our cellular power generators. But the jury is still out on whether natural selection set the mitochondrial thermostat.

Scientists have long used DNA from mitochondria (mtDNA) to trace human migration patterns. Because mtDNA is passed only from mother to child, it leaves a line of descent that can be traced through genetics. However, although the general pattern of human migration is fairly clear, scientists have struggled to explain pockets of regional variation within the mtDNA lineages.

Suspecting that the variations might represent adaptations to local environments, Douglas Wallace, a human geneticist at the University of California, Irvine, and his team analyzed mtDNA from 104 people representing all 18 major mtDNA lineages found in tropical (sub-Saharan Africa), temperate (Europe), and arctic (Siberia and North America) zones. They focused on 13 mtDNA proteins critical to heat and energy generation and discovered distinct mutation patterns for each group.

The arctic groups had the most mutations on a gene called ATP6. These changes would make the body's metabolism less efficient, thereby generating more body heat to protect against frigid weather, Wallace argues in the 23 December online Proceedings of the National Academy of Sciences. Tropical groups, on the other hand, had the most mutations in another gene, COI. Based on this gene's role in metabolism, Wallace argues that these mutations would boost efficiency and minimize body heat. People in temperate zones had a different pattern of mutations that should give rise to a metabolism intermediate to the other two groups. Statistical analysis indicates the mutations did not occur by chance, further supporting Wallace's argument that natural selection is at work.

Although the study is "highly suggestive and thought provoking," there's a lot more work to be done, says anthropological geneticist Joanna Mountain of Stanford University. For starters, there's no proof yet that the mutations really do play the functional roles Wallace describes, she says. "We also need to prove that survival and reproduction are enhanced by the mutations," adds Michael Crawford an anthropological geneticist at the University of Kansas in Lawrence. Moreover, he adds, "We can't characterize the whole world on the basis of 104 individuals."